Polymers prepared by cyclic ester polymerization and copolymerization such as poly(lactone)s and poly(lactide)s are interesting polymer systems in that they typically are biodegradable and bioassimilable. In addition, a number of monomers used to produce these polymers can be produced from biomass using enzyme technology or living organisms such as yeast.
Poly(lactic acid) (PLA), produced by ring-opening polymerization (ROP) of lactide (LA), is a leading biodegradable and biocompatible polyester with wide-ranging applications. Several well-defined Lewis acid catalysts have been developed for this reaction; prominent among them are metal alkoxides based on aluminum, zinc, and rare-earth metals, as well as organocatalysts. These known systems have successfully addressed one or more important factors in the ROP of LA, such as activity and catalyst toxicity. Enantioselectivity, in particular, has garnered much attention, and catalysts with exquisite control over PLA tacticity have been reported. Nevertheless, in most cases examples of highly selective catalysts that exhibit site control remain confined to aluminum salen complexes, which have low reactivity and functional-group tolerance.
There is a need for improved catalysts for the living polymerization and copolymerization of cyclic ester monomers. Improved catalysts can be, for example, less costly or highly reactive providing high turnover rates, enhanced stereochemical control or enhanced microstructure control, for example, by way of block copolymer self assembly and precise control over block ratio. Other desirable characteristics of improved catalysts include reduced toxicity, increased functional-group tolerance, increased stability or the ability to produce high molecular weight polymers and copolymers, particularly, with narrow molecular weight distribution. There is further a need for polymers and copolymers produced with such improved catalysts, specifically; there is a need for biodegradable polymers and copolymers with controlled microstructure.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.